Design: Power Export Facility plan

[kramsey458]

Adds design/POWER-EXPORT-PLAN.md: the design for the Power Export Facility, from the design discussion with Kyler. Plan only, no code.

• One building with two halves, like the Trading Post, one worker on each half; power networks never join except through one (always on in separate colonies).
• One direction per pair of colonies, 1-to-1 chains (A → B → C relays), no cycles.
• Sender's settings: Charge my batteries first (default on), Use my batteries for Player 2 (default off).
• Cost 20 Gears, 20 Planks, 20 Logs, 200 science. No trade at the building: payment happens at a Trading Post.
• Ctrl+P shows every colony's power networks, as Ctrl+L shows roads.
• A Power window on H (and a top-right button), like the colonies window.
• Native UI modelled on the rc31 Trading Post panel; VERIFY list for the mechanical system before building.

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